1. Field of the Invention
The invention relates to an agitator mill for the treatment of free flowing grinding stock, comprising a grinding receptacle, an interior wall of which defines a substantially closed grinding chamber; and an agitator unit, which is disposed rotatingly drivably in the grinding receptacle and is cup-shaped relative to a common central longitudinal axis and which comprises an annular cylindrical rotor, within which an interior stator is disposed, tightly joined to the grinding receptacle; an annular cylindrical exterior grinding chamber being formed between the interior wall of the grinding receptacle and an outer wall of the rotor, and an annular cylindrical interior grinding chamber, which is disposed coaxially within the exterior grinding chamber and is connected with the latter by way of a deflection chamber, being formed between an inner wall of the rotor and an outer jacket of the interior stator; the exterior grinding chamber, the deflection chamber and the interior grinding chamber constituting the grinding chamber partially filled with auxiliary grinding bodies; a grinding-stock supply chamber, which is disposed upstream of the exterior grinding chamber and opens into the latter in the direction of flow of the grinding stock, and a separating device, which is disposed downstream of the interior grinding chamber in the direction of flow of the grinding stock, being disposed approximately on the same side of the grinding receptacle for the grinding stock to pass through; and bypasses being provided in the agitator unit for the return of the auxiliary grinding bodies from the vicinity of the separating device into the vicinity of grinding-stock supply chamber, the bypasses connecting the end of the interior grinding chamber with the beginning of the exterior grinding chamber and--related to the direction of flow of the grinding stock--being disposed upstream of the separating device.
2. Background Art
An agitator mill of the generic type is known from U.S. Pat. No. 5,062,577. In this known agitator mill, pin-type agitator elements are mounted on the limiting walls of the exterior grinding chamber and at least on the inner limiting wall of the interior grinding chamber, by means of which alternate acceleration and deceleration of the auxiliary grinding bodies takes place, which leads to a turbulent-flow condition having a grinding and dispersing effect predominantly by impact. The grinding stock flows through a grinding-stock supply chamber, through a transition portion, past the bypasses into the exterior grinding chamber, and through the deflection chamber into the interior grinding chamber. The auxiliary grinding bodies circulate through the exterior grinding chamber, the deflection chamber, the interior grinding chamber and the bypasses back into the exterior grinding chamber or the transition portion opening into the latter, respectively. The grinding stock flows from the end of the interior grinding chamber to the separating device. The separating device does not considerably serve to separate the auxiliary grinding bodies on the one hand from the grinding stock on the other; nevertheless, the term separating device is used in this application too, because it is generally accepted in the technical language. As results from the above explanation, separating the auxiliary grinding bodies from the grinding stock takes place takes place already upstream of the separating device. The known agitator mill has been extraordinarily successful in practice.
An agitator mill is known from U.S. Pat. No. 4,304,362, of which the exterior grinding chamber on the one hand and the interior grinding chamber on the other each taper in the shape of a truncated cone, i.e. the cross-section of the grinding chamber is conical on each side of the central longitudinal axis of the rotor and the stator. The grinding stock flows through the agitator mill from the inside to the outside, i.e. it flows into the interior grinding chamber where the latter has its narrowest diameter, then it passes through the radially expanding interior grinding chamber, the deflection chamber and the radially expanding exterior grinding chamber. From there, it flows radially inwards through a chamber unilaterally defined by the agitator element and towards a separating device, through which the grinding stock is discharged. The inlet of a bypass is disposed downstream of this separating device, the inlet of this bypass being disposed radially within the separating device, i.e. disposed downstream of the latter. From there, the auxiliary grinding bodies flow through bypasses in the rotor into the starting portion of the interior grinding chamber. The limiting walls of the grinding chamber are smooth. The width of the grinding gap, i.e. the radial with of the grinding chamber, is constant; however, the distance towards the axis of rotation increases permanently. This results in the fact that the shear gradient increases from the inside to the outside along the path of the grinding stock. This means that either it is too low in the interior grinding chamber or too high in the exterior grinding chamber, which leads to irregular load exerted on the grinding stock. (The shear gradient is defined as the quotient of the speed of the rotating surface and the gap width.)